Decorative welts, or seaming lace as it is sometimes called, formed of plastic strip material is used extensively in forming decorative edges of upholstery, car panel trims and the like. The welts are often used at the joining edges of two panels to provide an embossed or decorative bead surface giving a finished appearance as when a gusset and panel are stitched together with the welt therebetween.
Conventional welts comprise an elongated bead portion which may or may not be reinforced and which has a flange extension which is actually sewed between a panel and gusset. The flange extension is provided with inverted V-shaped notches at predetermined intervals. The V-shaped notches permit a sewing machine operator to see through the welt flange at appropriate intervals matched to intervals in V-shaped positioning notches of a gusset and panel with which the welt is used. These notches are alignment means. The notches can be stamped in any pattern so long as the same pattern is used in both the panel and gusset. Thus when two parts are to be stitched together the sewing machine operator simply matches the notches at the beginning and during the stitching operation and the two pieces are thus perfectly matched when sewn. In this type of sewing operation, notches must be formed in the welting aligned with the notches in the panel and gusset. A suitably formed welt is first stitched to either the panel or the gusset and then that piece is stitched to the other. For example, if the welt is stitched to the panel, that piece is then stitched to the gusset. The welting must of course be provided with corresponding notches so that the operator can see through the flange and match the exact patterns of the gusset and panel which are then stitched carefully so that the notches line up.
A problem arises in forming notch patterns in the welt which conform to the notch patterns in alignment in the gusset and panels. In some cases the notches in the gusset and panels may vary for different items and particularly designed welts must be used with particularly designed notch patterns. Even when the notch pattern of the gusset and panel are known at the time of manufacture of the welt, it is difficult to form properly positioned notches. The welt is often an extruded material which has some stretch to it. Forming an exact notch pattern in an extruded welt which stretches to some extent during extrusion is difficult. Such difficulties are compounded by the fact that continuous lengths of welt are formed during extrusion and the stretching of the notching pattern can become cumulative.